Such a method is known from the published German Patent Application No. 31 48 441, which corresponds to U.S. Pat. No. 4,533,852, in which the molded body manufactured in this manner is a thermionic cathode. The substrate used in this method is formed in accordance with the desired cathode geometry. The substrate is, for example, a hollow cylinder and consists, for example of copper, nickel, iron, molybdenum or alloys thereof. A layered structure of a high-melting-point metal, for example, tungsten, molybdenum, tantalum, niobium, platinum, osmium, ruthenium, rhodium, rhenium, iridium or palladium, as well as optional emitter and doping materials, is provided on the substrate, for example, by chemical depositin from the gaseous phase (CVD method). As starting compounds of the metals are used, for example, the halides, carbonides, trifluorophosphanes, metallocenes, .beta.-diketonates and alcoholates thereof. The chemical deposition from the gaseous phase is caused, for example, by a plasma (for example in a plasma-activated CVD method commonly referred to in the art as the PCVD method). The subsequent removal of the substrate is carried out, for example, by etching, mechanical working and/or evaporation by heating.
A plasma deposition device is known from the published German Patent Application No. 31 17 252, which corresponds to U.S. Pat. No. 4,401,054, which comprises a plasma generator space as well as a sample space arranged so as to be separated therefrom, the plasma generator space being, for example, shaped and proportioned so that it satisfies the conditions of a microwave cavity resonator. The substrate to be coated is present outside the resonator in the sample space in which the plasma is extracted at least partly. The layers deposited on the substrate consist in particular of insulators, for example, silicon nitride and silicon dioxide or semiconductors, for example, silicon; electrically conductive materials, for example, molybdenum silicide and molybdenum, however, are also mentioned. Although the plasma in the sample space is accelerated under the effect of a diverging magnetic field, only very thin layers are produced at very low deposition rates.
The published British Patent Application No. 20 30 180 discloses an ion coating method, i.e. a method in which ions are formed and are accelerated towards the substrates to be coated by a high electric direct voltage field. The reaction space is formed by an electrically conductive tube and two insulating plugs with which the ends of the tube are closed. The inner wall of the tube constitutes the substrate to be coated. A plasma is produced in the reaction space by a direct current and/or high frequency discharge between the tube which is connected as the outer electrode and a thin tubular inner electrode which extends concentrically through the plugs in the whole reaction space. The electrical insulation between the tube and the inner electrode achieved by use of the insulating plugs is essential. The gaseous phase is decomposed by pyrolysis or chemical reduction, but microwaves or photolysis may also be used. In the case of the decomposition of the gaseous phase by microwaves, the microwaves should be radiated into the reaction space or be introduced into the reaction space via a waveguide. In this known method only a fraction of the microwave energy is used. Moreover, the microwave coupling ends after a short period and only very thin layers, i.e. approximately 1 .mu.m thick, are formed since the microwaves are coupled into the airtight-sealed reaction space implicitly via a dielectric, i.e. via the insulating plugs. The surface of the dielectric in the reaction space is inevitably also coated with the electrically conductive material so that further coupling is prevented.
An object of the invention is to deposit electrically conductive layers with optimum use of the microwave energy and to enable the production of longer-lasting coatings so that overall thicknesses of approximately 10 .mu.m or more are reached.